(1) Field of the Invention
The present invention relates generally to a winding method for a coil winding that can be used with the stator core of a three-phase DC brushless motor having a plurality of slots, and relates more particularly to a motor suitable for sensorless drive of pulse width modulation (PWM) operation.
(2) Description of Related Art
DC brushless motors use a sensorless drive method that does not require position detection elements in order to meet the strong demand for small, thin, low cost motors. Growing demand for low power consumption has also led to the use of PWM sensorless drive methods that achieve low power consumption by pulse width modulation of the motor drive current. Sensorless driving a DC brushless motor generally turns the motor by determining the appropriate commutation sequence by detecting the rotor position by detecting the point at which the back electromotive force of each phase generated proportionally to the motor speed, and the center tap voltage of the center tap where the three phase coils are connected in a star, are equal.
One method of detecting the specific rotor position supplies current to two phases, leaves the other phase non-energized, and detects the position of the output terminal by comparing the back electromotive force and the center tap voltage at the output terminal.
The motor terminal voltage varies greatly in PWM sensorless driving, however, as a result of PWM driving the motor output voltage. As a result, current flow to the motor coil has a time constant slope that is determined by the coil constant of the motor. A voltage known as the “induced voltage” occurs according to the time change in the current at the terminal of the non-energized phase. This is caused by the time change of the current and the mutual inductance representing the effect of each of the three phases on the other two phases. In conventional linear sensorless driving, the motor output voltage varies linearly, and linear current flows through the motor coils. The effect of mutual inductance is therefore zero.
Curve 101 in FIG. 7 represents the relationship between the rotor position and the induced voltage produced at the non-energized phase when a pulse voltage is applied from the U phase to the V phase as shown in FIG. 8. As shown by curve 101 in FIG. 7, the magnitude of the induced voltage varies according to the rotor position. Although the position of the rotor was fixed to measure the curve 101 shown in FIG. 7, the rotor is normally turning and the total voltage of this induced voltage plus the back electromotive force produced by rotor rotation is produced in the non-energized phase. This total voltage and the center tap voltage are compared to detect rotor position and switch the motor coil current to drive the motor.
If there is an imbalance in the number of windings or the method of winding the motor coils, there will also be an imbalance in the mutual inductance of the phases, and the induced voltage will therefore have a dc offset voltage as indicated by curve 100 in FIG. 7. When this dc offset voltage occurs, the rotor position may be wrongly detected in periods 1 in FIG. 7, that is, where the rotor should not normally be detected.
This induced voltage does not occur with conventional linear sensorless driving, however, and the rotor position can therefore be accurately detected without considering the winding method of the motor coil winding.
With PWM sensorless driving, however, an imbalance in the number of windings or the method of winding the motor coils can cause the rotor to be detected where the rotor is not actually positioned, resulting in startup failures or delayed starting.